The invention relates generally to X-ray tubes, and more particularly to structures and methods of assembly of a journal bearing increasingly used in x-ray tubes.
X-ray systems may include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, may be located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause variances in the attenuation of the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. The object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes include a cathode and an anode located within a high-vacuum environment. The anode structure is typically supported by one or more bearing members, such as ball bearings, and is rotated for the purpose of distributing the heat generated at a focal spot. Typically, an induction motor is employed to rotate the anode, the induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across an anode-to-cathode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed. Also, because the gantry assembly that includes the x-ray tube must spin with high rotational speed around a patient to provide good image quality, centripetal loads are high on the bearing assembly. This places stringent demands on the bearings and the material forming the anode structure, i.e., the anode target and the shaft supporting the target.
As ball bearings create issues with regard to wear and shorter useful lives for the bearing structure than is desired, in other constructions, a journal bearing, such as a liquid metal hydrodynamic bearing may be employed in lieu of ball bearings. In an x-ray tube journal bearing, the sleeve rotates relative to the shaft without any other moving parts in the bearing structure. To reduce wear between the shaft and the sleeve, a fluid, such as a liquid metal, is placed within the sleeve to lubricate the adjacent surfaces of the shaft and the sleeve, thereby limiting wear of the journal bearing assembly when in use. Advantages of liquid metal bearings include a high load capability and a high heat transfer capability due to an increased amount of contact area as compared to a ball bearing. Advantages also include low acoustic noise operation as is commonly understood in the art. Gallium, indium, or tin alloys are typically used as the liquid metal, as they tend to be liquid at room temperature and have adequately low vapor pressure, at operating temperatures, to meet the rigorous high vacuum requirements of an x-ray tube.
However, journal bearings also have certain shortcomings regarding their incorporation within x-ray tubes. In particular, in static or low speed journal bearing operation, the bearing fluid does not provide sufficient pressure between mating bearing parts to keep them from contacting. So, during coast-down, ramp-up, and assembly, there is significant possibility of rubbing between the mating inner and outer components of the sleeve and shaft forming the journal bearing. To limit the occurrences of wear as a result of this, the x-ray tube is operated in a manner that minimizes the number of ramp ups and coast or slow-downs that are performed, such that the bearing is nearly continuously operating. Continuous operation is ideal for the bearing but requires the system to stay on wasting energy and unnecessarily wearing out other system components.
As a result, it is desirable to develop a structure and method of operation for an x-ray tube journal bearing that can limit the contact of the shaft with the sleeve of the bearing during low sleeve rotational speeds, thus increasing the useful life of the journal bearing structure.